The invention relates to hydraulic lift mechanism and in particular to such mechanism as is required to serve intermittent alternating vertical displacement of a load, wherein the load may be of various magnitudes within the capacity of the mechanism. Such conditions exist for hydraulically operated cranes and hoists such as fork lifts, and for hydraulic elevators.
Conventional cranes and hoists employ a prime mover such as a diesel engine or one of various types of electric motor, depending upon the design capacity of the involved lift system, and the rated power of the prime mover is conservatively selected for assured handling of the maximum rated load of the system. In most cases, the system further requires a gearbox, a speed reducer, a pulling drum and a safety brake. Illustratively, for example, a crane with a 1-ton lift capacity (at one meter/second) requires a prime mover of 15 horsepower, and a crane with 10-ton lift capacity (at the same one meter/second) requires a prime mover of 150 horsepower.
Conventional electric-motor driven elevators are known as traction elevators. They rely on cable suspension of an elevator car from one side of a drive sheave at the upper end of the elevator shaft, with a counterweight suspended by the same cable from the other side of the drive sheave, the counterweight being designed to at least offset the weight of the car, so that theoretically the prime mover need only supply power adequate to handle loads up to the live-load capacity of the system. As a practical matter, however, such elevators must meet a requirement for fast initial acceleration from a dead start; this requirement calls for relatively high current-handling capacity so that the prime mover must be of substantially greater capacity, e.g., three times the capacity required to move the load after its initial acceleration to design running speed.
By contrast, the car of a conventional hydraulic elevator is at the upper end of an elongate vertical drive piston, operating in an elongate cylinder beneath the low end of the elevator shaft. There is no equivalent to the counterweight of a traction elevator. The prime mover for upward displacement of the car is an electric motor to drive a pump, for drawing hydraulic fluid from a sump reservoir and delivering the same via suitably controlled valve means to the head end of the cylinder; descent proceeds gravitationally via suitably controlled valve means in a throttling-flow connection to the sump from the head end of the cylinder. The net result is that prime-mover power must always be of sufficient capacity to elevate maximum load on the system, at specified conditions of speed and initial acceleration.
As far as I am aware, Bailey, U.S. Pat. No. 269,994 of 1883 is alone in suggesting that a hydraulic accumulator could serve to counterbalance a rotary pump-driven hydraulic-elevator system on the basis of an average live-load on the load-positioning hydraulic cylinder of the system. But Bailey's system was susceptible to irretrievable leaks (outside his system) via the reversibly driven gear pump he proposed to use between the accumulator and the load-positioning cylinder, so that if the elevator car were to assuredly hold stationary at a given floor-landing elevation, it would be necessary to close two shut-off valves, one on each side of the Bailey gear pump, in that the Bailey disclosure of an unillustrated brake to hold the pump cannot be a means to hold the car, due to the unavoidable leakage. Moreover, the Bailey disclosure provides no suggestion of means to replenish hydraulic fluid lost by leakage.